Electrostatic charge accumulation may lead to electrostatic discharge which may present a hazard in many situations. Electrostatic discharge may cause ignition of flammable substances. Unlike other potential ignition sources, electrostatic charge accumulation may be due to normal operations. For example, liquid fuel flow through a tube may generate electrical charge in the fuel and/or tube that accumulates because the fuel and/or tube are electrically insulating. Additionally, in fuel handling, transport, and storage, liquid fuel may form a vapor or aerosol in use. If exposed to oxygen and a sufficiently intense electrostatic discharge under uncontrolled conditions, the fuel may ignite unexpectedly.
Prevention and characterization of electrostatic discharge may be important to industries which use or produce flammable environments. Industrial examples include fuel production, fuel transportation, vehicle operation, mining operations, chemical processing, metal fabrication, power plant construction and operation, and operations which involve combustible particulate such as sawdust, metal, flour, and grain.
Propagating brush discharges, one of many types of electrostatic discharges, are known to be incendive in many situations. A propagating brush discharge is a type of electrostatic discharge that occurs on a dielectric surface (also called an insulating surface). If a dielectric surface is sufficiently insulating and has a sufficiently high dielectric strength (breakdown field strength), opposite electrical charge can accumulate in layers on opposite sides of the surface. Generally, this bi-layer of opposite charge accumulation occurs when the dielectric layer has one side in contact with a source of mobile charge carriers (typically a grounded metal surface) and the other side is exposed to a charge source (e.g., friction due to material flow). Such an arrangement may allow the exposed side of the dielectric layer to accumulate a large charge density (e.g., 0.1-10 mC/m2 (millicoulombs per square meter)). A propagating brush discharge occurs when the formerly isolated sides of the dielectric layer become electrically connected. Electrical connection between the two sides of the dielectric layer may be formed by a point in the dielectric layer that experiences dielectric breakdown, by flashover on the surface to an exposed portion of the source of mobile charge carriers, or by mechanical contact with a grounded component (or other source of mobile charge carriers). The discharge forms a radiating branch pattern (a radial Lichtenberg form) that drains some or all of the charge from the surfaces.
Propagating brush discharges typically arise when the electrical potential between the opposite sides of the dielectric layer is several thousand volts (e.g., greater than 4 kV (kilovolts)), the dielectric layer is thin (e.g., less than 10 mm (millimeters)), and the accumulated charge density is relatively large (e.g., greater than 0.1 mC/m2). The total energy released by a propagating brush discharge may be greater than 1 mJ (millijoules), may occur in less than 10 μs (microseconds), and may have a peak current of greater than 10 A (amperes). Charge accumulation that may lead to propagating brush discharges typically involves rapid mechanical flow of insulating powders or liquids through insulating conduits or conduits including an insulating internal surface, or exposure to high electric fields.
In traditional propagating brush discharge testing, a dielectric layer is charged to produce a bi-layer of static electrical charge on opposing surfaces and a propagating brush discharge is prompted by moving a grounded electrode towards the dielectric layer. The grounded electrode typically has a large bulbous tip and is moved by a person using a long handle. As the grounded electrode is brought close enough to the dielectric layer, the electrical conditions may be sufficient to trigger a propagating brush discharge. The time of initiating a propagating brush discharge is variable and reliant on particular conditions of the test. For example, the approach speed of the grounded electrode, the electrode shape, the electrode position relative to the dielectric layer, the material of the dielectric layer, the voltage of the charged surfaces, and the environmental conditions (e.g., air pressure, humidity, etc.) may affect the precise point in time when a propagating brush discharge may be triggered.